Zoom lenses of conventional cameras operate so that the magnification of an object standing at a certain fixed distance is varied during zooming ("normal zooming mode").
When a photographer wants to use a zoom lens to keep up with a moving object (i.e., a proceeding or receding object) while maintaining the magnification constant ("constant magnification zooming mode"), it has been necessary to simultaneously operate the zooming and focusing in order to keep the moving object infocus. Practically, however, it is very difficult to deliberately operate both the zooming ring and focusing ring simultaneously.